1. Field of the Invention
This invention relates to photoimageable epoxy resin coating compositions and composite articles of dry film photoresist made using the said compositions that are useful in the fabrication of electronic components, micro-electromechanical system (MEMS) components, micromachine components, microfluidic components, micro total analysis system (μ-TAS) components, microreactor components, electroconductive layers, lithographie, galvanoformung, abformung (LIGA) components, forms and stamps for microinjection molding and microembossing, screens or stencils for fine printing applications, MEMS and electronic packaging components, and printed wiring boards that can be processed by ultraviolet (UV) lithography. Suitable electronic component applications include dielectric layers, insulation layers, and photoconductive wave circuits. Other uses include resin substrates for other photoimageable layers and in the construction of array structures for biochemical analysis and in the construction of cell growth platforms for biological materials. Still other suitable applications may include the fabrication of buried channel and air-bridge structures used, for example, in microfluidic or optical devices.
2. Brief Description of Art
Photoimageable coatings are currently used in a wide variety of semiconductor and micromachining applications. In such applications, photoimaging is accomplished by exposing the coating on a substrate to patterned radiation thereby inducing a solubility change in the coating such that the exposed or unexposed regions can be selectively removed by treatment with a suitable developer composition. The photoimageable coating (photoresist) may be either of the positive or negative type, where exposure to radiation either respectively increases or decreases the solubility in the developer.
The most common photoimageable coatings useful in microelectronic applications are liquid compositions comprising a film forming resin, a photoactive compound, and a solvent. These compositions may be applied to a substrate either directly in liquid form and then dried to form a coating on the substrate or they may be first formed into a composite film comprising a substantially dried coating of the liquid resist on a carrier film such that when the coated side of the composite film is contacted with the substrate under the action of heat and pressure, the photoimageable coating is adhered to the substrate and the carrier film is then removed leaving the photosensitive layer on the substrate. Photoimageable composite films of the type described are commonly referred to in the art as dry film photoresists. Depending on the application, dry film photoresists may offer advantages over liquid photoresists especially when the coated substrate is not compatible with the solvents present in the liquid resist or when the substrate, either for technical or economic reasons, cannot be baked under the conditions of time and temperature necessary to remove the solvent.
Conventional positive resists based on diazonaphthoquinone-novolac chemistry are not well-suited to applications requiring film thicknesses greater than about 10 microns. This thickness limitation is caused by the relatively high optical absorbance of the diazonaphthaquinone-type (DNQ) photoactive compounds at wavelengths in the near-ultraviolet region of the optical spectrum (350-450 nm) which are typically used to expose the resist. Also, DNQ-type photoresists possess limited contrast, or differential solubility, of the exposed vs. unexposed resist in a developer solution which results in relief image sidewalls that are sloped rather than vertical. Optical absorption necessarily reduces the radiation intensity as radiation traverses from the top to the bottom of the film, such that if the optical absorption is too high, the bottom of the film will be underexposed relative to the top, causing a sloped or otherwise distorted profile of the developed image. Nevertheless, DNQ resist formulations are available for use at film thicknesses up to 100 microns, but at a great increase in the required exposure dose.
A negative, spin-coated, thick-film photoimageable composition of the chemically amplified type, which has a very low optical absorbance at wavelengths in the 350-450 nm range has been described in the literature [N. LaBianca and J. D. Gelorme, “High Aspect Ratio Resist for Thick Film Applications”, Proc. SPIE, vol. 2438, p. 846 (1995)]. High aspect ratio (>10:1) photoimaging was demonstrated in 200 micron thick films. This resist comprises a solution in a casting solvent of a highly branched, multifunctional epoxy bisphenol A-novolac resin, EPON® SU-8 from Resolution Performance Products, and a photoacid generator (PAG) such as CYRACURE® UVI 6974 from Dow Chemical which consists of a mixture of arylsulfonium hexafluoroantimonate salts. The resulting photoresist formulation may be spin coated or curtain coated onto a wide variety of substrates, pre-baked to evaporate solvent, leaving a solid photoresist coating of one hundred microns or greater thickness which may be photoimaged by exposure to near-ultraviolet radiation through a patterned photomask using contact, proximity, or projection exposure methods. Subsequent immersion of the imaged layer in a developer solution dissolves away the unexposed regions, leaving behind a high resolution, negative-tone relief image of the photomask in the film.
EPON® SU-8 resin is a low molecular weight, multi-functional epoxy oligomer that has several characteristics making it advantageous for high aspect ratio photoimaging in thick films: (1) it has a high average epoxide functionality, (2) a high degree of branching, (3) high transparency at wavelengths of 350-450 nm, and (4) the molecular weight is sufficiently low as to allow preparation of high solids coating compositions. The high functionality and branching result in efficient crosslinking under the influence of strong acid catalysts, while the high transparency allows uniform irradiation through thick films, making the resist capable of forming images with aspect ratio of greater than 10:1 at film thicknesses of greater than 100 microns. In fact, only high epoxy functionality and a high degree of branching will provide high aspect ratio structures with straight sidewalls.
Suitable photoacid generators based on sulfonium or iodonium salts are well-known and have been extensively discussed in the literature [see for ex. Crivello et al., “Photoinitiated Cationic Polymerization with Triarylsulfonium Salts”, Journal of Polymer Science: Polymer Chemistry Edition, vol. 17, pp. 977-999 (1979).] Other useful PAGs with appropriate absorbance include the carbonyl-p-phenylene thioethers as described in U.S. Pat. Nos. 5,502,083 and 6,368,769. Additionally, sensitizers such as 2-alkyl-9,10-dimethoxyanthracenes or various naphthalene, peryl or pyryl compounds can be added to the formulation or incorporated into the PAG as described in U.S. Pat. No. 5,102,772.
Negative photoresists based on the above disclosed compositions which are suitable for spin-coating are sold by MicroChem Corp., Newton, Mass., USA and are used commercially, especially in the fabrication of MEMS devices. For example, a product typically offered by MicroChem, “SU-8 50” can be spin-coated at 1000-3000 rpm to produce films of thickness in the range of 30-100 microns, which after exposure and development; can produce images having an aspect ratio greater than 10:1 at film thicknesses greater than 100 microns. Higher or lower solids versions extend the film thickness range obtainable by a single coat process to less than 1 micron and above 200 microns. Casting of the solution can result in films of 1 to 2 mm or more in thickness.
U.S. Pat. No. 6,391,523 assigned to MicroChem Corp. discloses a composition useful for a thick film negative resist comprising a mixture of at least one epoxidized polyfunctional bisphenol A formaldehyde novolak resin and at least one photoacid generator in a coating solvent, a majority amount of said coating solvent being cyclopentanone and suitable co-solvents being dimethylformamide, N-methyl pyrrolidinone, ketonic solvents such as, cyclohexanone, heptanone, methylamyl ketone and methyl isopropyl ketone, cyclic ethers such as 1,3-dioxolane and tetrahydofuran, hydroxylic polar solvents such as tetrahydrofurfuryl alcohol, ethyl lactate, propyleneglycol methylether, propyleneglycol methylether acetate, methylmethoxy propionate, 2-ethoxyethyl acetate, propylene carbonate, 2-methoxyethanol, ethylethoxypropionate and the like.
U.S. Pat. No. 6,716,568 assigned to MicroChem Corp., which issued on Apr. 6, 2004 discloses a photoimagable composition suitable for use as a negative photoresist comprising about 75% to about 95% by weight of at least one epoxidized polyflnctional bisphenol A formaldehyde novolak resin; about 5% to about 25% by weight of at least one polyol reactive diluent; and at least one photoacid generator in the amount from about 2.5 to about 12.5 parts per hundred parts of resin and reactive diluent; dissolved in a sufficient amount of a casting solvent. The named casting solvents include gamma-butyrolactone, cyclopentanone, propylene glycol methyl ether acetate, cyclohexanone and methyl ethyl ketone. The preferred polyol reactive diluent is a polycaprolactone polyol.
U.S. Pat. Nos. 4,882,245 and 4,940,651 disclose a photoimageable cationically polymerizable composition for use in printed circuit boards which consists of a mixture of up to 88% epoxidized bisphenol A formaldehyde novolac resin with average epoxide functionality of eight and a reactive diluent which serves as a plasticizer, and a cationic photoinitiator. Reactive diluents disclosed were mono- or di-functional cycloaliphatic epoxides, preferably at 10-35% by weight solids. Also disclosed are the use of these formulations as permanent layers, where the layer is not removed from the substrate, but becomes a part of the structure, such as a dielectric layer on a printed circuit board.
U.S. Pat. Nos. 5,026,624, 5,278,010, and 5,304,457 disclose a photoimageable, cationically polymerizable fire retardant composition suitable for use as a solder mask, which consists of a mixture of the 10-80% by weight condensation product of bisphenol A and epichlorohydrin, 20-90% by weight of epoxidized bisphenol A formaldehyde novolac resin, and 35-50% by weight of epoxidized glycidyl ether of tetrabromobisphenol A, with 0.1-15 parts per hundred by weight of a cationic photoinitiator. Curtain coating, roll coating, and wound wire rod coating were used as methods of coating.
U.S. Pat. No. 4,256,828 discloses a photopolymerizable composition based on an epoxy resin of functionality greater than 1.5, a hydroxyl-containing additive, and a photoacid generator. The hydroxyl-containing additive is reported to increase flexibility and decrease shrinkage for coatings of up to 100 microns thickness.
U.S. Pat. No. 5,726,216 describes a toughened epoxy resin system and the methods for making and using such a system in electron beam radiation curable applications. The main difficulty they claim to overcome is the brittleness of the radiation cured epoxy resins where the resins for many structural, non-structural or other consumer products must have sufficient toughness and impact resistance to endure many years of harsh service. They disclose a wide variety of toughening agents that can be incorporated into the base epoxy resin or mixture, which may include SU-8 resin. Effectiveness of the claimed invention with respect to increased toughness was measured by fracture toughness and flexural modulus. The toughening agents claimed constitute a variety of thermoplastics, hydroxy-containing thermoplastic oligomers, epoxy-containing thermoplastic oligomers, reactive flexibilizers, elastomers, rubbers, and mixtures thereof. However, the compositions of U.S. Pat. No. 5,726,216 were formulated as coatings imaged with non patterned electron beam radiation and no reference was made to the photoimaging characteristics of these formulations when exposed to imaged ultraviolet, X-ray, or electron beam radiation.
There have been many other prior art proposals for different photoimageable compositions including many that use epoxies. Examples of these can be found as referenced in U.S. Pat. No. 5,264,325. Here it is further taught that the resist material must be formulated such that it can be applied by coating methods, for example spin coating, which requires certain rheological properties. In addition, the composition must have the properties of providing sufficient transmission of the exposing radiation so as to photolyze the photoinitiator through the thickness of the film, and the resist must possess appropriate physical and chemical properties to withstand the application, such as solder or ink resistance or toughness, without significant degradation, or loss of adhesion. If it is to be used for other purposes, such as an etch photoresist, other properties may be required.
U.S. Pat. No. 4,882,245 also describes the application of these materials as a dry film photoresist when coated onto a carrier medium such as Mylar film. Numerous other U.S. patents and other references teach the preparation and use of dry film photoresists. The U.S. patents include U.S. Pat. Nos. 3,496,982; 3,782,939; 4,193,797; 4,193,799; 4,247,616; 4,576,902; 4,624,912; 4,672,020; 5,077,174; 5,120,633; 5,145,764; 5,262,281; 5,405,731; 6,066,889; 6,204,456; 6,462,107; and 6,528,218.
Generally, dry film photoresist compositions are made from a liquid photoimagable composition which is coated onto a first substrate using one of several conventional coating techniques. A substantial portion of the solvent in the liquid photoimagable composition is removed by heating or other suitable processing to form a dry film photoresist layer on the first substrate. Later, the dry film is imaged and developed according to conventional processing either on this first substrate or after being transferred onto a second substrate. These imaged and dry films can be then removed from that substrate by conventional stripping or resist removal processing (in which case this resist layer is a temporary resist) or hardened and caused to become part of the end use application (in which case this resist layer is a permanent resist).
While these numerous references teach various resist formulations, there is still a need for better resist formulations for modem dry films applications, including those useful in the microfluidic device field. The present invention offers a solution to that need for those better resists for those modem day film applications.